Effects of α+γ Intercritical and γ Single-Phase Annealing on Texture Evolution in Cold-Rolled Dual-Phase Steel Sheets

Abstract

Dual-phase (DP) steel sheets composed of both soft ferritic and hard martensitic phases are typical advanced high strength steel sheets applicable to a variety of automobile parts. The crystallite texture of the steel sheet is one of the important factors that influence press formability. However, the texture of the martensite itself in DP steels has not been discussed since the texture was generally measured by the X-ray diffraction method, which does not distinguish the texture of martensite from that of ferrite. The objective of this study is to investigate the effects of intercritical and γ single-phase annealing on the texture evolution in DP steels by a newly-developed analysis method using Electron Back-Scatter Diffraction (EBSD) to obtain the texture of each phase separately. The chemical composition of the steel used was 0.1%C-1.2%Si-2.3%Mn-0.1%Ti (mass%). The 1st-annealing was carried out at 948K, which is below the Ac1 temperature, in order to finish recrystallization after hot and cold rolling so as to focus on the transformation texture evolution itself. The steels were subsequently annealed both at 1123K in the intercritical region and at 1223K in the γ single-phase region to obtain DP microstructures with approximately 40% volume fraction of martensite. The overall texture including martensite in the case of intercritical annealing was similar to the initial texture before annealing, while the texture became randomized in the case of γ single-phase annealing. Moreover, our unique EBSD analysis method clearly showed that the textures of the martensite themselves were close to those of ferrite under the two annealing conditions.